The purpose of this proposal is to better understand the role of cholesterol in normal embryonic development. The role of cholesterol has recently been highlighted by the discovery that mutations of the sterol delta-7 reductase (DHCR7), the last enzyme in the cholesterol biosynthesis pathway, cause the Smith-Lemli-Opitz Syndrome (SLOS, MIM 270400). SLOS is a relatively common multiple congenital abnormality syndrome, affecting embryonic development of brain, heart, limbs, intestine and lungs, as well as a number of other organs. Sterol delta-7 reductase is predicted to be a 9 transmembrane-domain spanning integral membrane protein localized to the endoplasmic reticulum and catalyzes the conversion of 7-dehydrocholesterol to cholesterol. Understanding the biology of cholesterol metabolism during embryonic development is therefore of some importance. In addition, understanding the molecular biology of sterol delta-7 reductase, how the mutations disrupt this enzyme and gaining a better understanding of this enzyme may allow us, in the future, to devise better strategies to treat patients with Smith-Lemli-Opitz syndrome. To accomplish this, we have combined the powerful approaches of genetics, animal models, and cell biology to elucidate structure-function and biological relationships. We have developed a mouse model for SLOS, using targeted disruption of the murine dhcr7 gene. We will use tissue-specific in vivo complementation to abrogate the neonatal lethality of the homozygous knock out mice. Additionally, molecular characterization of both the knockout mice, as well as our in vitro studies will help us better understand the mechanistic processes involved in both the pathogenesis of SLOS, as well as the normal role of cholesterol in embryogenesis.